Help with replacing crankset and BB

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MountainSurfer
Posts: 56
Joined: 9 May 2021, 8:16pm

Help with replacing crankset and BB

Post by MountainSurfer »

I have a new gravel bike with Shimano GRX 810 front and rear mech (the gravel specific components that are equivalent to Ultegra). However the only non-GRX componentry the bike came with is the Praxis Alba 2x crankset and Praxis M24 BB86, press fit bottom bracket.

It is impossible to get rid of chain rub from the FD in multiple chain positions, and I have to engage the high trim (when in big ring) when the RD is in the 5th sprocket from bottom in the cassette because of chainrub on the inside of the FD, rather than when it is in the 9/10/11th sprockets as should be usual ie. much earlier than normal. I have used multiple sources to adjust the FD, including the Shimano manual and multiple good sources online. Two different shops have had a go and they haven't been able to do any better.

A couple of Youtube reviews of the GRX groupset (including GCN) mentioned that because the FD is 2.5mm further outboard from the frame (to help with tyre clearances on gravel bikes), the GRX crankset is built to accommodate this difference to road mechs. They specifically stated that if you get the crankset, you need the GRX FD, not a non-GRX one. I can only assume that the opposite also applies, and this mismatch is the reason chainrub is incessant in multiple positions despite all adjusting (ie. Praxis crankset is just a regular crankset that doesn't accommodate for this 2.5mm outboard difference as compared with road FDs, so chain is constantly 2.5mm closer to the frame/inside of FD than it should be).

So I'm thinking of replacing the crankset with either the GRX810 (48/31 Ultegra equivalent) or 610 (46/30, 105 equiv), both Hollowtech II and about £60 difference in price. But I really can't figure out if they'll be compatible with the current BB, or if I need to buy a new BB as well? Haven't had much luck finding online, so wondering if any of the knowledgeable people here can help out?
Tompsk
Posts: 195
Joined: 6 Nov 2014, 9:35am

Re: Help with replacing crankset and BB

Post by Tompsk »

If the bike is new the original shop should be responsible to sort the problem rather than you having to spend money to fix it.

Edit, typo.
Jamesh
Posts: 2963
Joined: 2 Jan 2017, 5:56pm

Re: Help with replacing crankset and BB

Post by Jamesh »

If your replacing the chainset I would replace the BB too.

If it's a pressfit then adaptors are available to convert.

Cheers James
keyboardmonkey
Posts: 1120
Joined: 1 Dec 2009, 5:05pm
Location: Yorkshire

Re: Help with replacing crankset and BB

Post by keyboardmonkey »

Unless you bought it from a private seller I would be tempted to return the bike. Not fit for purpose and all that stuff.
alexnharvey
Posts: 1922
Joined: 10 Jan 2014, 8:39am

Re: Help with replacing crankset and BB

Post by alexnharvey »

If your theory is correct about the 2.5mm crank offset it seems reasonable to ask for correct parts to be fitted to resolve the issue or return the whole bike.
TheBomber
Posts: 520
Joined: 16 Feb 2020, 8:18pm

Re: Help with replacing crankset and BB

Post by TheBomber »

Couple of thoughts though I don’t have any direct experience of the components involved:

- your theory sounds plausible but have you measured the chainline of the Praxis?
- bottom brackets often have various spacers within them that can be swapped from one side to the other, thereby changing the chainline
- wouldn’t it be cheaper to change the FD? A road 11 speed should work with the levers and the narrower chainline. Maybe one of the shops you’ve used could test one out on there for you?
MountainSurfer
Posts: 56
Joined: 9 May 2021, 8:16pm

Re: Help with replacing crankset and BB

Post by MountainSurfer »

I called up the shop for a different issue the week I got it, they just said that these are the parts that Giant issue on their bikes as standard so it's nothing to do with us. The chain fitted was too short, had to replace it. They said if I sent them a picture of the bike with the old chain, they would forward to Giant and see if they will refund me the cost of new chain.

I live in Manchester, the shop is in Inverness. I just can't see getting a resolution from them given that it's a stock Giant bike with parts that they fit as standard for this bike all around the world? Giant aren't going to pay for new crankset given that they fit this for their bikes as standard.

Re. returning the bike, yes I guess that may be a compromise. But then this was basically the only gravel bike out of all models I had shortlisted that was available in my size with specs I was largely happy with. Given there are no bikes available at the moment, I'm not going to be able to find another one that ticks the boxes for several months.
keyboardmonkey
Posts: 1120
Joined: 1 Dec 2009, 5:05pm
Location: Yorkshire

Re: Help with replacing crankset and BB

Post by keyboardmonkey »

In ordinary times I still think you should be able to return the bike as your contract is with the bike shop - they can’t dodge that responsibility IMHO. However, it’s something of a seller’s market as you say, especially as the shop is miles away. I only hope the dodgy crank/BB were fitted for reasons of cost or availability and that you can do a straight swap. Best of luck anyway.
MountainSurfer
Posts: 56
Joined: 9 May 2021, 8:16pm

Re: Help with replacing crankset and BB

Post by MountainSurfer »

TheBomber wrote: 24 Jul 2021, 10:09am Couple of thoughts though I don’t have any direct experience of the components involved:

- your theory sounds plausible but have you measured the chainline of the Praxis?
- bottom brackets often have various spacers within them that can be swapped from one side to the other, thereby changing the chainline
- wouldn’t it be cheaper to change the FD? A road 11 speed should work with the levers and the narrower chainline. Maybe one of the shops you’ve used could test one out on there for you?
Re. the chainline, I haven't measured anything, though I would have no clue what to reference any measurements to

Re. FD, yes this would be easier. However my OCD tendencies wouldn't like that as then I'd end up with TWO non-GRX components in the groupset, whereas if I changed the crankset, everything would now be GRX haha :shock:
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: Help with replacing crankset and BB

Post by Jdsk »

MountainSurfer wrote: 24 Jul 2021, 11:15amRe. the chainline, I haven't measured anything, though I would have no clue what to reference any measurements to
https://sheldonbrown.com/chainline.html
Although that might be getting a bit out of date for modern components.

https://www.parktool.com/blog/repair-he ... e-concepts

Jonathan
MountainSurfer
Posts: 56
Joined: 9 May 2021, 8:16pm

Re: Help with replacing crankset and BB

Post by MountainSurfer »

Does anyone know the correct BB I should use for this?

I’ve found that BBR60 threaded is compatible with crankset but the current BB86 is press fit, so I’m assuming my frame doesn’t have threads. Will need an appropriate press fit one?
Jamesh
Posts: 2963
Joined: 2 Jan 2017, 5:56pm

Re: Help with replacing crankset and BB

Post by Jamesh »

Yes you need a press fit BB and GRX chainset.

Otherwise you could get press fit adaptor and use a ht2 BB and HT2 GRX chainset??

Cheers James
MountainSurfer
Posts: 56
Joined: 9 May 2021, 8:16pm

Re: Help with replacing crankset and BB

Post by MountainSurfer »

Have spent quite a bit of time this evening trying to find the definitive answer.

The current BB is a Praxis M24 BB86 pressfit (AKA BB90/BB92)

Praxis instructions state this is for 41mm frame shell diameters and 86.5mm width ( https://praxiscycles.com/wp-content/upl ... L-REVD.pdf )

Park Tool state this about the BB86 standard - PF41 (ALSO KNOWN AS BB86, BB89.5, BB92, BB107, BB132, SHIMANO® PRESS-FIT, PF24) ( https://www.parktool.com/blog/repair-he ... erminology )

I've found this Shimano BB92 pressfit BB, which seems to be one of the "AKA"s of the BB86 standard, and specifically states it's made for Shimano HollowTech II cranks and is compatible with 41mm cup diameter and 86.5mm width shells ( https://tinyurl.com/5e45jm5h )

This is the crankset ( https://tinyurl.com/h92aczxh )

So I'm hoping my logic stands up to reason and this is the one to go for!

I'm hoping there's no funny business with spacers required as that would mess things up as if I get it wrong first time (no spacers when needed, or spacers when not needed) then I understand if you remove a press fit BB it should not be used again.

Just need to get a cheap satisfactory tool(s) for removing old and installing new and then all set!
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531colin
Posts: 16034
Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: Help with replacing crankset and BB

Post by 531colin »

Measure the existing chainline.
Otherwise you could buy a whole lot of stuff and end up back precisely where you are now.
Chainline is the distance between the centre of the frame (ie. the centre of the seat tube or downtube) and the middle of the gap between the 2 chainwheels.
It might be easier to measure from the middle of the gap between the chainwheels to the near side and the far side of the frame tube, and do the maths, because its quite difficult to know exactly where the centre of a round tube is.

The people who make your existing BB https://praxiscycles.com/product/m24-bb86-bb90-bb92/ say it has different bearing sizes each side, and will fit their crankset and one other (sram?) one.
I have never examined any of these things, but it would be well to check.
There is an instruction manual you can download, from a quick glance it talks about "dust shields" of different thicknesses rather than spacers as such.
MountainSurfer
Posts: 56
Joined: 9 May 2021, 8:16pm

Re: Help with replacing crankset and BB

Post by MountainSurfer »

So I've done a measurement (difficult because deraileur in the way and frame is a weird shape, but can go back later and see if I get the same number for validity) and it's about 45mm from centre of frame (a curved rhomboid shape that is bigger at the BB level and tapers as it goes up to the seatpost) at the level of the chainrings to midpoint between big and little ring.

But excuse my ignorance, what do I do with this measurement?
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